1. Field of the Invention
The invention relates to electronic circuits, and in particular to a Gilbert cell mixer and a method thereof.
2. Description of the Related Art
Mixer circuits are commonly employed in telecommunication systems to convert one frequency to another. One frequency conversion technique known as direct conversion is popular. Direct conversion refers to techniques whereby an incoming RF signal is received and converted directly to baseband signals without Intermediate Frequency (IF) components. A current trend is to integrate radio frequency (RF) transceiver functionality onto a single die. It is important to reduce circuit dimension and die size, which results in reduction in manufacturing costs.
FIG. 1 is a circuit schematic of a conventional IQ mixer, comprising Gilbert cells 10a and 10b, each comprising a separate emitter degeneration inductor. IQ Mixer 1 modulates input signals RFin+ and RFin− with oscillation signals LO_I± and LO_Q±. Gilbert cell 10a constitutes cross-coupled differential amplifiers, and BJT transistors 110a and 112a convert input voltage signals RFin+ and RFin− respectively to current to feed into a switching network including transistors 102a, 104a, 106a, and 108a. Transistors 102a, 104a, 106a, and 108a mix input signals RFin+ and RFin− with local oscillation signals LO_I+ and LO_I− to produce an in-phase output voltage VI across intermediate frequency signals IFI+ and IFI−. Similarly, Gilbert cell 10b mixes input signals RFin+ and RFin− with local oscillation signals LO_Q+ and LO_Q− to produce a quadrature output voltage VQ. Employment of separate degeneration inductors La and Lb results in large circuit dimensions for conventional IQ mixers.
Thus a need exists for a mixer and method to provide a compact circuit arrangement without affecting circuit performance.